Metal cutting or "machining" is recognized to be one of the most important and most widely used processes is manufacturing. Among the common machining operations are shaping, planing, milling, facing, broaching, grinding, sawing, turning, boring, drilling and reaming. Some of these processes, such as sawing, operate on both the external and internal surfaces of the workpiece, while others operate only on the internal (e.g. reaming) or external (e.g. milling) surfaces of the workpiece. These various processes are described in detail in DeGarmo, Materials and Processes in Manufacturing, 3rd edn., 1969), especially in chapter 16, "Metal Cutting".
The measure of productivity of a given machining operation is determined by the total amount of metal removed from the workpiece in a given period of time. To this end a wide variety of materials have been used or suggested as cutting tools. These are commonly classified as tool steels, high speed steels, cast non-ferrous alloys, sintered carbides and ceramics. (There are also some limited applications for diamonds). The commonly measured parameters of cutting tool performance are cutting speed, depth of cut, feed rate and tool life. Each of these prior art cutting tool materials is deficient in one or more of these parameters. Tool steel, high speed steel and cast non-ferrous alloys all have critical temperature limitations which restrict their cutting speed to relatively low rates, as measured in feet per minute (fpm) or meters per minute (m/min). Typically high speed steels are restricted to 100-225 fpm (30-70 m/min) for cutting steel and 250-300 fpm (75-14 90 m/min) for cutting non-ferrous materials. The cast non-ferrous alloys will operate at up to about twice those rates. The carbide materials, such as tungsten carbide, improve on the cutting speed rates of the steels by a factor of 2-5, particularly when the carbides are coated. However, the carbides are not as tough as the steels and are susceptible to impact breakage. This severely limits their usage to applications where impact is a factor, such as in making interrupted cuts or in machining hard workpieces.
Ceramic materials, such as alumina, have been found to produce cutting tools which can operate at much higher speeds than the conventional steel and carbide cutting tools. For instance, cutting speeds of 500-1400 fpm (150-430 m/min) for steel cutting have been reported. Tool life with the ceramics, however, has been a serious problem, because the ceramics are even more brittle and less tough than the carbides. Of particular concern has been the tendency of the ceramic materials to fracture catastrophically and unexpectedly when subjected to impact. Thus, while cutting speeds have been high for the ceramic materials, it has been possible to operate them only at quite low feed rates, much lower than those used for the steels and carbide cutting tools.
It has thus been found that productivity, which is a function of both cutting speed and feed rate, is relatively low for all prior art types of cutting tools. The steel and carbide tools, while having high feed rates, have relatively low cutting speeds. Conversely, the ceramics, while having high cutting speeds, operate only at low feed rates. Productivity, determined as total amount of metal removal for a given time period, therefore remains relatively low regardless of the type of cutting tool used.
References relating to the use of various ceramics as cutting tools include U.S. Pat. No. 4,543,343 to Iyori et al which describes use of a ceramic comprised of alumina, zirconia and titanium carbide together with titanium boride. Another reference is U.S. Pat. No. 4,366,254 which describes a cutting tool ceramic comprised of alumina and zirconia together with carbides, nitrides or carbo-nitrides of Group IVB and VB metals.
There have been some teachings that silicon carbide fiber reinforced ceramics can be used in various machine parts. Examples shown have included heat exchangers, molds, nozzles, turbines, valves and gears; see Japanese patent Nos. 59-54680 and 59-102681. Such disclosures, however, are not particularly pertinent to the cutting tool invention described herein, since such parts are not subject to impact stresses as part of their normal service environment. No mention is made of improved toughness or impact resistance nor are such properties of interest in the articles described.
It has also been disclosed that fracture toughness in ceramics can be improved by incorporation of silicon carbide whiskers into the ceramics. Two papers by Becher and Wei have described mechanisms for increase in toughness as related to whisker content and orientation; see "Toughening Behavior is SiC Whisker Reinforced Alumina", Comm. Am. Cer. Soc. (December 1984) and "Transformation Toughened and Whisker Reinforced Ceramics", Soc. Auto. Engrs., Proc. 21st Auto. Tech. Dev. Mtg., 201-205 (March, 1984). See also Wei U.S. Pat. No. 4,543,345. These papers, however, deal only with thermal and flexural applications and do not address anything with respect to machining processes. It would therefore be highly advantageous to have a tool which operates at the high cutting speeds of the ceramics while also allowing the high feed rates found with the steels and carbides. Such cutting tools would yield productivities significantly higher than any of the prior art tools.